Implantable sensors can monitor bone-healing process

Biomedical engineers at the Rensselaer
Polytechnic Institute have created an implantable sensor that
can be placed in the site of recent orthopaedic surgery to transfer
data about how the body is healing. The sensor could provide a more
accurate, cost effective and less invasive way to monitor and
diagnose the body post-surgery.

The current way of monitoring a patient's recovery after an
orthopaedic procedure relies on X-rays and MRIs. These new sensors could give surgeons detailed, real-time
information from the actual surgery site, which could help to
better understand potential complications.

The sensors are four millimetres in diameter and 500 microns
thick. They look like small coils of wire and are attached to
commonly-used musculoskeletal implants such as rods, plates or prostheses. Once implanted,
the sensor can monitor and transmit data about the load, strain, pressure,
or temperature of the healing surgery site. The sensor is scalable,
tunable, and easy to configure so that it may be incorporated into
many different types of implantable orthopaedic devices. They
don't need a battery: instead, they are powered by an external
device used to capture the data.

The sensors work by measuring internal displacement. This
internal displacement can be made sensitive to force, pressure or
temperature depending on how the sensors are tuned. Theoretically a
number of different sensors tuned for different measurements could
be implanted.

Eric Ledet, assistant professor in the department of Biomedical
Engineering at Rensselaer, told Wired.co.uk that the physics behind
the sensor is similar to a tuning fork. With a tuning fork, you add
mechanical energy by banging it against something. It then
resonates at a characteristic frequency, which we hear as a
sound.

"Our sensors are also resonators, but they are energised by radio
frequency energy. When you subject them to a radio frequency
field using an antenna, they resonate at a characteristic
frequency. That resonant frequency is modulated by force or
pressure or temperature. So we add radio frequency energy to the
sensor, allow it to resonate, then we "listen" to its resonant
frequency with an antenna. Passive resonator sensors are not new,
but ours have no electrical connections which makes them very
simple and very robust."

The device they use to "listen" to the resonance is an
"off-the-shelf network analyser (made by Agilent) with some custom electronics". The system created the
radio frequency field to energise the sensors and then also listens
for the resonant frequency.

The team has filed for patent protection for the new sensor.
They currently make each one by hand and are investigating methods
for mass production.